Machines for the cold working of bar stock have been known. For example, bars of round or profiled section have been clamped in a stationary fixture on the one hand and in a rotary chuck on the other hand, the fixture and chuck being attached to the machine bed in a suitable manner and having jaws that are coupled to a continuous drive by a clutch to be engaged are required. In a conventional machine of this type, a shifting device is employed for disengaging the clutch. A linkage is provided between a selector fork and a thrust member for actuation. In order to disengage the clutch, the shifting device which is guided along cheek bars must be returned manually into its initial position. This design is rather complex, involving a heavy structure. It is, moreover, prone to misoperation and to deterioration because of inevitable wear and tear of the shifting device that is exposed to excessive strain.
The use of another conventional bar twisting machine is also impeded by a complicated construction. For drawing a bar during twisting, a longitudinally movable gripper is operated by a double action fluid cylinder whose control must be combined with an electric motor drive for a rotary chuck. Manufacture of such a heavy machine is expensive. In addition, the apparatus requires a large space and considerable skill on the part of the operator.